Two pyelocalyceal systems associated with single or double ureters. The ureters empty separately into the bladder or fuse to form a single ureteral orifice. Can be unilateral or bilateral and occur in 15% of the population, F>M.

During embryogenesis, if a single ureteral bud bifurcates before bifurcation of the ampulla, a duplex kidney results with bifid pelvis or bifid ureter. If two ureteral buds arise from the Wolffian duct, a duplex kidney results with complete ureteral duplication.